A look at balljoint failure
#1
A look at balljoint failure
Recently we had a balljoint fail on our 944 race car at Sebring in turn 17. Luckily nothing was hit and the driver was able to bring the car to a stop outside of the racing line and by some miracle was able to limp the car into pit-in and get the car to our garage.
I'lll give more details in my writeup for our Sebring 2017 race when I get around to updating that.
I popped the top of the ball joint pin out of the knuckle and brought it to work so I could put it under the microscope.
Up close, you can see it was, as expected, a fatigue failure with the flaw starting as a semi-elliptical surface crack and working its way though the pin until the crack straightened out and grew more rapidly. Complete failure happened about 2/3rds to 3/4s of the way through and this is where it completely unzipped as shown by the yellow line.
Its tough to tell the orientations from the photos, but the initiation side is actually the outward side of the ball joint (facing the wheel), which makes sense as the bending moment of the wheel is putting the outside of the pin in tension.
This potentially could have been picked up with penetrant inspection, but the inspection would likely cost more than the ball joint rebuild kit and you'd have to drop the control arm anyway. Better to just replace them early.
I'lll give more details in my writeup for our Sebring 2017 race when I get around to updating that.
I popped the top of the ball joint pin out of the knuckle and brought it to work so I could put it under the microscope.
Up close, you can see it was, as expected, a fatigue failure with the flaw starting as a semi-elliptical surface crack and working its way though the pin until the crack straightened out and grew more rapidly. Complete failure happened about 2/3rds to 3/4s of the way through and this is where it completely unzipped as shown by the yellow line.
Its tough to tell the orientations from the photos, but the initiation side is actually the outward side of the ball joint (facing the wheel), which makes sense as the bending moment of the wheel is putting the outside of the pin in tension.
This potentially could have been picked up with penetrant inspection, but the inspection would likely cost more than the ball joint rebuild kit and you'd have to drop the control arm anyway. Better to just replace them early.
#2
Race Car
Stock arm and ball joint pin....?
Two active threads on recent problems with these.
"My roll center correction" and "Racing".
Did the driver notice an off center steering wheel right before it went...?
Do you think you have a solution...?
T
Two active threads on recent problems with these.
"My roll center correction" and "Racing".
Did the driver notice an off center steering wheel right before it went...?
Do you think you have a solution...?
T
#3
Drifting
Can you elaborate a little for those of us who are interested, but unknowleged?
Is the failure from lateral force from the tires on the road trying to decamber the wheel while cornering? (you mentioned it was a bending moment failure) How is there a bending moment on the pin if it's a ball joint and not a shear force? Do you suspect it was binding at all?
Is the failure from lateral force from the tires on the road trying to decamber the wheel while cornering? (you mentioned it was a bending moment failure) How is there a bending moment on the pin if it's a ball joint and not a shear force? Do you suspect it was binding at all?
#4
Race Car
I have pics of a snapped extended pin in the other thread but if I added one, a head on shot of the broken cross section, it looks just like this.
Surprising to me though...., Sebring, main stressed tire is LF not RF. T1 is pretty tough on RF but T17 is brutal and notoriously bumpy.
I see a tire rub mark on the door and I wonder if another car clipped this car's tire/wheel while it was exposed in a turn.
T
#5
Is the failure from lateral force from the tires on the road trying to decamber the wheel while cornering? (you mentioned it was a bending moment failure) How is there a bending moment on the pin if it's a ball joint and not a shear force? Do you suspect it was binding at all?
The load is a combination of shear load and tensile bending.
I don't suspect the pin was binding. We check them before every race and this one felt fine beforehand.
#6
It was a stock arm, not using the longer pin. We run the car low, but not that low. About 5" from ground to rocker panel.
The door donut came from Daytona 2016.
#7
Race Car
The balljoints were not equal age. We had already rebuilt the drivers side, but as the passenger side had no play we put that rebuild off to focus on other projects.
No solution other than to know that they can fail at any time, so replace them early. We know there are 8 14 hour races on the ball joints + however many hours from prior owners and however many years of sitting around.
No solution other than to know that they can fail at any time, so replace them early. We know there are 8 14 hour races on the ball joints + however many hours from prior owners and however many years of sitting around.
Can't think of a safe place to lose one at Sebring (been lucky there so far) except maybe T7, reduced speed and run off towards hotel. If you lost one between 13 and 15, lost steering control and hit a jutting end wall, that'd be big trouble.
Lost results are one thing but I'm thinking the safety aspect.
Was this LeMons or one of the organizations like it...?
T
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#8
Ticking time bomb and no telling when they will go off, that's the problem.
Can't think of a safe place to lose one at Sebring (been lucky there so far) except maybe T7, reduced speed and run off towards hotel. If you lost one between 13 and 15, lost steering control and hit a jutting end wall, that'd be big trouble.
Lost results are one thing but I'm thinking the safety aspect.
Can't really tell because your paint is so dull ( ) but usually, the fender is a write off.
Was this LeMons or one of the organizations like it...?
T
Can't think of a safe place to lose one at Sebring (been lucky there so far) except maybe T7, reduced speed and run off towards hotel. If you lost one between 13 and 15, lost steering control and hit a jutting end wall, that'd be big trouble.
Lost results are one thing but I'm thinking the safety aspect.
Can't really tell because your paint is so dull ( ) but usually, the fender is a write off.
Was this LeMons or one of the organizations like it...?
T
Turn 15-16 has a bit of runoff room to the outside but your point stands - there arent many good places and unfortunately you don't get to pick the time when in fails.
This was with Chumpcar, so yes, like lemons.
#9
Drifting
We've been tossing the idea around of going with high strung control arms for our chumpcar and just taking the points on them. We also used rattle can for our car. Krylon red is reasonably close to guards red!
#10
We did get many events out of ours and admittedly we don't know how old the arms were when we got the car. Unless you are going REALLY low, (in which case, I'd use the offset pin), I'd just run the stock arms with a fresh rebuild and you will probably be okay unless you are entering every race they do in a year.
We only take points for the coilover conversion kit, which isn't much and we still have a lot of room, but I'd still rather save that for future engine modifications.
#11
Drifting
So back to the ball joint question again... If the failure is a moment couple applied by the ball joint at one end and the rigidly attached spindle at the other, would it not stand to reason that increasing the length of the pin would increase the internal moment in the pin?
#12
Rennlist Member
Markus Blazak used to have a collection of pictures of various balljoint/control arm failures. Unfortunately, I searched here this morning and can't find the threads or a website for him anymore.
I have seen autopsy evidence of socket failures, pins bending, and breaks below the pinch bolt.
This failure right at the pinch bolt groove is why Porsche eliminated the full circumference groove on the Cup/Motorsport arms (why they never did this on the later production arms, no idea).
I have seen autopsy evidence of socket failures, pins bending, and breaks below the pinch bolt.
This failure right at the pinch bolt groove is why Porsche eliminated the full circumference groove on the Cup/Motorsport arms (why they never did this on the later production arms, no idea).
#13
Rennlist Member
So back to the ball joint question again... If the failure is a moment couple applied by the ball joint at one end and the rigidly attached spindle at the other, would it not stand to reason that increasing the length of the pin would increase the internal moment in the pin?